Notice that if a=3m+12⋅5m+10 and b=5m+59m+1 are both integers, then their product,
ab=(3m+1)(5m+5)(2⋅5m+10)(9m+1)
is also an integer.
Collecting now a factor of two in the numerator, this expression can be rewritten as
(3m+1)(5m+5)2(5m+5)(9m+1)=3m+12(9m+1).
Let us now see for which m this expression is an integer. Noting that 9m+1=(3m+1)2−2⋅3m, we can further transform the product ab into the form
ab=2⋅3m+1(9m+1)=2⋅3m+1(3m+1)2−2⋅3m=2(3m+1)−43m+13m
which is an integer if and only if 43m+13m is an integer.
We now prove that 2<43m+13m<4.
Indeed, since m≥1, we have 3m≥3, and therefore 2⋅3m+2<2⋅3m+2⋅3m<4⋅(3m+1); dividing these three terms by 3m+1 we then find 2<43m+13m<4 as desired.
Therefore, if 43m+13m is an integer, it must necessarily be equal to 3, since it is strictly between 2 and 4.
Imposing this equality we obtain 4⋅3m=3⋅(3m+1)⇒3m=3⇒m=1.
Therefore, if there exists an m with the required property, it must be equal to 1. On the other hand, it is easy to verify that 31+12⋅51+10=5 and 51+591+1=1 are both integers. We can therefore conclude that m=1 is indeed a solution and is the only one.